Look in a row, column or box if there are x number of cells containing x possible numbers, where all the cells has the same possible numbers. Meaning, if a cell has only 2 and 3 as possible numbers, look for another cell in the same row, column or box which also has only 2 and 3 as possible numbers. If this exists, these cells must contain the number 2 or 3 as solution and we can remove 2 and 3 from the row, column or box.

1

9

2

7

8

6

6

7

9

4

1

2

8

4

7

9

9

5

2

6

7

3

9

1

1

9

6

4

9

1

2

8

4

1

5

9

6

6

2

8

9

1

5

3

7

4

Here you see that R3 has two cells which have 3 and 5 as possible solutions. Therefore we can remove 3 and 5 from the other cells in R3, in this case 3 and 5 from R3C4 and 3 from R3C5.

1

9

2

345

34

7

8

6

35

35

6

7

358

38

9

4

1

2

8

35

4

12356

236

126

7

35

9

347

378

9

1468

5

1468

2

38

37

245

58

6

7

248

3

9

58

1

2357

1

35

28

9

28

6

4

357

9

357

35

346

3467

46

1

2

8

37

4

1

238

2378

28

5

9

6

6

2

8

9

1

5

3

7

4

This doesn't work only for 2 numbers in 2 cells. You can do it for the same numbers in 3 cells, 4 cells etc. This sudoku can be solved using DS and SiSo.
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